Journal Club

Read. Discuss. Understand.

Repeat.

Courant Journal Club 2014/15: Water Vapor - Radiation - Climate

Water defines our world, and water vapor is arguably the single most important atmospheric constituent for our climate,
linking radiation, chemistry, and dynamics.
Our goal with this journal club series is to better understand the various aspects of how water vapor determines, and is determined by,
the state of the atmosphere.

Program

The momentum budget residual X = (X, Y) is estimated with objectively analyzed soundings taken during the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) intense observing period (IOP; November 1992–February 1993) to study the effects of convective momentum transport (CMT) over the western Pacific warm pool. The time series of X and Y exhibit multiscale temporal behavior, showing modulations by the Madden–Julian oscillation (MJO) and other disturbances. The power spectra of X, Y, and ITBB (an index of convective activity) are remarkably similar, showing peaks near 10, 4–5, and 2 days, and at the diurnal period, suggesting a link between deep cumulus convection and the acceleration– deceleration of the large-scale horizontal motion, via CMT, which is being modulated by various atmospheric disturbances. The temporal behavior of X and Y can be described as fractals from 1/4 to ~20 and from 1/4 to ~6 days, respectively. Their fractal characteristics are reflected in the very large standard deviations around the small IOP means. From the analyses of the quantities uX/|u| , vY/|v| , and v·X, the IOP-mean vertical distributions of the frictional force due to subgrid-scale eddies and the rate of kinetic energy transfer (E = -v · X) are determined. The frictional deceleration and downscale energy transfer take place in a deep tropo- spheric layer from the surface to 300 hPa. In addition, a concentration of large friction and energy transfer exists in a layer just below the tropopause, suggesting the contribution of momentum detrainment from the top of deep cumuli. The IOP-mean frictional deceleration and downscale energy transfer in the lower troposphere are ~0.5–1.0 m s-1 day-1 and ~1.0 x 10^4 m2 s-3, respectively. The product of eddy momentum flux with the large-scale vertical wind shear shows that the momentum transport is, on the average, downgradient; that is, kinetic energy is converted from the large-scale motion to convection and turbulence.

Surface temperatures increase at a greater rate over land than ocean in simulations and observations of global warming. It has previously been proposed that this land–ocean warming contrast is related to different changes in lapse rates over land and ocean because of limited moisture availability over land. A simple theory of the land–ocean warming contrast is developed here in which lapse rates are determined by an assumption of convective quasi-equilibrium. The theory predicts that the difference between land and ocean temperatures increases monotonically as the climate warms or as the land becomes more arid. However, the ratio of differential warming over land and ocean varies nonmonotonically with temperature for constant relative humidities and reaches a maximum at roughly 290 K.
The theory is applied to simulations with an idealized general circulation model in which the continental configuration and climate are varied systematically. The simulated warming contrast is confined to latitudes below 50° when climate is varied by changes in longwave optical thickness. The warming contrast depends on land aridity and is larger for zonal land bands than for continents with finite zonal extent. A land–ocean temperature contrast may be induced at higher latitudes by enforcing an arid land surface, but its magnitude is relatively small. The warming contrast is generally well described by the theory, although inclusion of a land–ocean albedo contrast causes the theory to overestimate the land temperatures. Extensions of the theory are discussed to include the effect of large-scale eddies on the extratropical thermal stratification and to account for warming contrasts in both surface air and surface skin temperatures.

During summertime, monsoons and subtropical anticyclones shape precipitation and regional circulation patterns across the globe. In state-of-the-art climate models, the average response of the Asian monsoon cyclone, Pacific ocean anticyclone and jet stream to global warming is weak and responses of different models are diverse. Here we use a suite of simulations with atmospheric general circulation models with prescribed sea surface temperatures to separate the circulation responses to direct radiative forcing and indirect sea surface temperature warming. We find that the two contributions oppose each other. Using idealized aquaplanet simulations, we show that the different circulation responses are directly connected to the opposite responses of land–sea thermal contrast to the two forcing components. This tug of war on the circulation response to global warming is analogous to the seasonal response to insolation, which involves opposite land–sea thermal contrasts and circulation patterns governed by quasi-equilibrium thermodynamics and stationary-wave dynamics. We conclude that it is important to distinguish weak circulation responses to global warming that arise owing to compensating effects that are robust and physically understood from those that are associated with genuine uncertainty. We note that compensation places fundamental limits on the detection and attribution of circulation responses to global warming.

A large-eddy simulation (LES) was performed for a Hector thunderstorm observed on 30 November 2005 over the Tiwi Islands. On that day, ice particles reaching 19-km altitude were measured. The LES developed overshooting updrafts penetrating the stratosphere that compared well with observations. Much of the water injected in the form of ice particles sublimated in the lower stratosphere. Net hydration was found with a 16% increase in water vapour. While moistening appeared to be robust with respect to the grid spacing used, grid spacing on the order of 100 m may be necessary for a reliable estimate of hydration.

The double-Intertropical Convergence Zone (ITCZ) problem, in which excessive precipitation is produced in the Southern Hemisphere tropics, which resembles a Southern Hemisphere counterpart to the strong Northern Hemisphere ITCZ, is perhaps the most significant and most persistent bias of global climate models. In this study, we look to the extratropics for possible causes of the double-ITCZ problem by performing a global energetic analysis with historical simulations from a suite of global climate models and comparing with satellite observations of the Earth’s energy budget. Our results show that models with more energy flux into the Southern Hemisphere atmosphere (at the top of the atmosphere and at the surface) tend to have a stronger double-ITCZ bias, consistent with recent theoretical studies that suggest that the ITCZ is drawn toward heating even outside the tropics. In particular, we find that cloud biases over the Southern Ocean explain most of the model-to-model differences in the amount of excessive precipitation in Southern Hemisphere tropics, and are suggested to be responsible for this aspect of the double-ITCZ problem in most global climate models.

Idealized dynamical theories that employ a convective quasi-equilibrium (QE) treatment for the diabatic
effects of moist convection have been used to explain the location, intensity, and intraseasonal evolution of monsoons. This paper examines whether observations of the earth’s regional monsoons are consistent with the assumption of QE. It is shown here that in local summer climatologies based on reanalysis data, maxima of free-tropospheric temperature are, indeed, nearly collocated with maxima of subcloud equivalent potential temperature, \theta_{eb}, in all monsoon regions except the North and South American monsoons. Free-tropospheric temperatures over North Africa also exhibit a strong remote influence from the South Asian monsoon. Consistent with idealized dynamical theories, peak precipitation falls slightly equatorward of the maxima in ueb and free-tropospheric temperature in regions where QE seems to hold. Vertical structures of temperature and wind reveal two types of monsoon circulations. One is the deep, moist baroclinic circulation clearly seen in the South Asian monsoon. The other is of mixed type, with the deep moist circulation superimposed on a shallow dry circulation closely associated with boundary layer temperature gradients. While the existence of a shallow dry circulation has been documented extensively in the North African monsoon, here it is shown to also exist in Australia and southern Africa during the local summer. Analogous to moist QE theories for the deep circulation, the shallow circulation can be viewed in a dry QE framework in which shallow ascent occurs just equatorward of the peak boundary layer potential temperature, \theta_b, providing a unified system where the poleward extents of deep and shallow circulations are bounded by maxima in \theta_{eb} and \theta_b, respectively.

The diurnal cycle of tropical convection is investigated with global cloud imagery constructed from 11μm radiance measurements taken aboard six satellites. Four harmonics of the diurnal cycle are resolved in the cloud imagery with about 50 km horizontal resolution. To isolate deep convective activity from other processes which cause diurnal fluctuations in longwave radiance, an index of deep convective activity is constructed by thresholding to brightness temperatures less than 230 K. Significant diurnal amplitude of deep convection is found only over tropical landmasses. Over the tropical oceans the diurnal cycle is weak and is barely discernible from the background red spectrum of convective variance. Nonetheless, oceanic convection exhibits a systematic diurnal fluctuation with maximum intensity in the early morning. Nocturnal subsidence along the cloud-free equator is postulated to play a role in forcing diurnal variation in the intertropical convergence zones. Other mechanisms are also implied to contribute as a similar early morning maximum in deep convection is seen even where no adjoining cloud-free regions occur. The diurnal cycle of deep convection is found to be organized on planetary scales predominantly in nonmigrating modes (i.e., modes with phase speeds not equal to that of the Sun). The nonmigrating modes are postulated to be produced by the nonuniform distribution of convective centers. The nonmigrating, zonally symmetric component is found to be especially large. While the role of this mode in forcing diurnal variations of the tropospheric circulation (e.g., the Hadley cell) is questionable, its role in modulating the ionospheric electric potential is well established. Discrepancies with the traditional notion that cloud-to-ground lightning is the dominant mechanism by which the Earth is negatively charged are discussed. In particular, the probable role of nonlightning-producing clouds (e.g., the stratiform portion of convective complexes) in diurnally modulating the negative charge on Earth is discussed.

We show here that stratospheric water vapor variations play an important role in the evolution of our climate. This comes from analysis of observations showing that stratospheric water vapor increases with tropospheric temperature, implying the existence of a stratospheric water vapor feedback. We estimate the strength of this feedback in a chemistry-climate model to be +0.3 W/(m(2)⋅K), which would be a significant contributor to the overall climate sensitivity. One-third of this feedback comes from increases in water vapor entering the stratosphere through the tropical tropopause layer, with the rest coming from increases in water vapor entering through the extratropical tropopause.

(Pauluis (2011)): The impact of water vapor on the production of kinetic energy in the atmosphere is discussed here by comparing two idealized heat engines: the Carnot cycle and the steam cycle. A steam cycle transports water from a warm moist source to a colder dryer sink. It acts as a heat engine in which the energy source is the latent heat of evaporation. It is shown here that the amount of work produced by a steam cycle depends on relative humidity and is always less than that produced by the corresponding Carnot cycle.
The Carnot and steam cycles can be combined into a mixed cycle that is forced by both sensible and latent heating at the warm source. The work performed depends on four parameters: the total energy transport; the temperature difference between the energy source and sink; the Bowen ratio, which measures the partitioning between the sensible and latent heat transports; and the relative humidity of the atmosphere. The role of relative humidity on the work produced by a steam cycle is discussed in terms of the Gibbs free energy and in terms of the internal entropy production.

(Laliberte et al. (2015)): Incoming and outgoing solar radiation couple with heat exchange at Earth’s surface to drive weather patterns that redistribute heat and moisture around the globe, creating an atmospheric heat engine. Here, we investigate the engine’s work output using thermodynamic diagrams computed from reanalyzed observations and from a climate model simulation with anthropogenic forcing. We show that the work output is always less than that of an equivalent Carnot cycle and that it is constrained by the power necessary to maintain the hydrological cycle. In the climate simulation, the hydrological cycle increases more rapidly than the equivalent Carnot cycle. We conclude that the intensification of the hydrological cycle in warmer climates might limit the heat engine’s ability to generate work.

This paper offers a critical review of the topic of cloud–climate feedbacks and exposes some of the underlying reasons for the inherent lack of understanding of these feedbacks and why progress might be expected on this important climate problem in the coming decade. Although many processes and related parameters come under the influence of clouds, it is argued that atmospheric processes fundamentally govern the cloud feedbacks via the relationship between the atmospheric circulations, cloudiness, and the radiative and latent heating of the atmosphere. It is also shown how perturbations to the atmospheric radiation budget that are induced by cloud changes in response to climate forcing dictate the eventual response of the global-mean hydrological cycle of the climate model to climate forcing. This suggests that cloud feedbacks are likely to control the bulk precipitation efficiency and associated responses of the planet’s hydrological cycle to climate radiative forcings.

Convectively coupled equatorial waves (CCEWs) control a substantial fraction of tropical rainfall variability. Their horizontal structures and dispersion characteristics correspond to Matsuno's (1966) solutions of the shallow water equations on an equatorial beta plane, namely, Kelvin, equatorial Rossby, mixed Rossby-gravity, and inertio-gravity waves. Because of moist processes, the tilted vertical structures of CCEWs are complex, and their scales do not correspond to that expected from the linear theory of dry waves. The dynamical structures and cloud morphology of CCEWs display a large degree of self-similarity over a surprisingly wide range of scales, with shallow convection at their leading edge, followed by deep convection and then stratiform precipitation, mirroring that of individual mesoscale convective complexes. CCEWs have broad impacts within the tropics, and their simulation in general circulation models is still problematic, although progress has been made using simpler models. A complete understanding of CCEWs remains a challenge in tropical meteorology.

The climate feedbacks in coupled ocean–atmosphere models are compared using a coordinated set of twenty-first-century climate change experiments. Water vapor is found to provide the largest positive feedback in all models and its strength is consistent with that expected from constant relative humidity changes in the water vapor mixing ratio. The feedbacks from clouds and surface albedo are also found to be positive in all models, while the only stabilizing (negative) feedback comes from the temperature response. Large intermodel differences in the lapse rate feedback are observed and shown to be associated with differing regional patterns of surface warming. Consistent with previous studies, it is found that the vertical changes in temperature and water vapor are tightly coupled in all models and, importantly, demonstrate that intermodel differences in the sum of lapse rate and water vapor feedbacks are small. In contrast, intermodel differences in cloud feedback are found to provide the largest source of uncertainty in current predictions of climate sensitivity.

Based on reanalysis data for the years 1980–2001 from the European Centre for Medium-Range Weather Forecasts (ERA-40 data), a climatology of tropospheric zonal-mean water vapor fields and fluxes in isentropic coordinates is presented. In the extratropical free troposphere, eddy fluxes dominate the meridional flux of specific humidity along isentropes. At all levels, isentropic eddy fluxes transport water vapor from the deep Tropics through the subtropics into the extratropics. Isentropic eddy fluxes of specific humidity diverge near the surface and in the tropical and subtropical free troposphere; they converge in the extratropical free troposphere. Isentropic mean advective fluxes of specific humidity play a secondary role in the meridional water vapor transport in the free troposphere; however, they dominate the meridional flux of specific humidity near the surface, where they transport water vapor equatorward and, in the solstice seasons, across the equator. Cross-isentropic mean advective fluxes of specific humidity are especially important in the Hadley circulation, in whose ascending branches they moisten and in whose descending branches they dry the free troposphere.

Near the minima of zonal-mean relative humidity in the subtropical free troposphere, the divergence of the cross-isentropic mean advective flux of specific humidity in the descending branches of the Hadley circulation is the dominant divergence in the mean specific humidity balance; it is primarily balanced by convergence of cross-isentropic turbulent fluxes that transport water vapor from the surface upward. Although there are significant isentropic eddy fluxes of specific humidity through the region of the subtropical relative humidity minima, their divergence near the minima is generally small compared with the divergence of cross-isentropic mean advective fluxes, implying that moistening by eddy transport from the Tropics into the region of the minima approximately balances drying by eddy transport into the extratropics. That drying by cross-isentropic mean subsidence near the subtropical relative humidity minima is primarily balanced by moistening by upward turbulent fluxes of specific humidity, likely in convective clouds, suggests cloud dynamics may play a central role in controlling the relative humidity of the subtropical free troposphere.

Recent progress is reviewed in the understanding of convective interaction with water vapor and changes associated with water vapor in warmer climates. Progress includes new observing techniques (including isotopic methods) that are helping to illuminate moisture-convection interaction, better observed humidity trends, new modeling approaches, and clearer expectations as to the hydrological consequences of increased specific humidity in a warmer climate. A theory appears to be in place to predict humidity in the free troposphere if winds are known at large scales, providing a crucial link between small-scale behavior and large-scale mass and energy constraints. This, along with observations, supports the anticipated water vapor feedback on climate, though key uncertainties remain connected to atmospheric dynamics and the hydrological consequences of a moister atmosphere. More work is called for to understand how circulations on all scales are governed and what role water vapor plays. Suggestions are given for future research.

Water vapor is the dominant greenhouse gas, the most important gaseous source of infrared opacity
in the atmosphere. As the concentrations of other greenhouse gases, particularly carbon dioxide,
increase because of human activity, it is centrally important to predict how the water vapor distribution
will be affected. To the extent that water vapor concentrations increase in a warmer world, the climatic effects
of the other greenhouse gases will be amplified. Models of the Earth’s climate indicate that this is an
important positive feedback that increases the sensitivity of surface temperatures to carbon dioxide
by nearly a factor of two when considered in isolation from other feedbacks, and possibly by as much as a factor
of three or more when interactions with other feedbacks are considered. Critics of this consensus
have attempted to provide reasons why modeling results are overestimating the strength of this feedback.
Our uncertainty concerning climate sensitivity is disturbing. The range most often quoted for the equilibrium
global mean surface temperature response to a doubling of CO2 concentrations in the atmosphere is 1.5C to 4.5C.
If the Earth lies near the upper bound of this sensitivity range, climate changes in the twenty-first century
will be profound. The range in sensitivity is primarily due to differing assumptions about how the Earth’s
cloud distribution is maintained; all the models on which these estimates are based possess strong water vapor feedback.
If this feedback is, in fact, substantially weaker than predicted in current models, sensitivities in the upper half
of this range would be much less likely, a conclusion that would clearly have important policy implications.
In this review, we describe the background behind the prevailing view on water vapor feedback and some of the arguments
raised by its critics, and attempt to explain why these arguments have not modified the consensus within the
climate research community.

Some potential candidates

Schneider & Soden (2007): The Global Circulation of the Atmosphere - Chapters from various authors

Bony et al. (2006): How Well Do We Understand and Evaluate Climate Change Feedback Processes?

Dufresne & Bony (2008): An Assessment of the Primary Sources of Spread of Global Warming Estimates from Coupled Atmosphere–Ocean Models

Dufresne & Bony (2008): An Assessment of the Primary Sources of Spread of Global Warming Estimates from Coupled Atmosphere–Ocean Models

Fueglistaler et al. (2009): Tropical Tropopause Layer

Kang et al. (2009): The Tropical Response to Extratropical Thermal Forcing in an Idealized GCM: The Importance of Radiative Feedbacks and Convective Parameterization

Schneider et al. (2010): Water vapor and the dynamics of climate changes